1. Field of the Invention
This invention relates generally to extendible booms usable in gantries and other equipment for lifting heavy objects. More particularly, the invention relates to an extendible boom having a locking mechanism, which is configured to prevent unintended lowering of the boom and which includes a stabilizer arrangement, which coordinates operation of multiple components of the locking assembly.
2. Discussion of the Related Art
Extendible booms are well-known for a variety of heavy lift applications. Depending on the application and on the configuration of the booms, they can be used either individually or in gantries, which lift loads using two or more extendible booms operating in conjunction with one another. The typical boom of this type includes a stationary base and a lifting leg including at least first and second boom sections extending above the base. The first section is telescopically extendible relative to the second section using one or more hydraulic lift cylinders coupled to the first section. The second section may be stationary or may itself be telescopically extendible relative to a third section using the same multi-stage lift cylinder used to extend the first section or using a dedicated single-stage or multi-stage cylinder. In some booms, all extendible sections are xe2x80x9cpoweredxe2x80x9d because they are permanently coupled to at least one lift cylinder. The maximum stroke of such a boom is equal to the maximum stroke of the boom""s cylinder(s). A boom of this type is disclosed, for example, in U.S. Pat. No. 4,381,839 to Engler et al. In other booms, at least one extendible section is a xe2x80x9cmanualxe2x80x9d section because it can be fixed to an adjacent boom section after it is extended so as to permit the associated lift cylinder(s) to be retracted without retracting the manual section. This permits the cylinder(s) to subsequently be coupled to the adjacent section and then lift the two sections as a unit. In this manner, a load can be lifted to a height that exceeds the maximum stroke of the cylinder(s). A boom of this type is disclosed, for example, in U.S. Pat. No. 5,865,327 to Johnston (the Johnston ""327 patent) and is marketed by JandR Engineering, Inc. under the Tradename xe2x80x9cLift-N-Lock.xe2x80x9d
The typical extendible boom relies on the integrity of the system""s hydraulic pressure system to lift the load and to maintain the load in its lifted position. If the system experiences hydraulic pressure loss due, e.g., to a rupture of a hose, hydraulic pressure source failure, or hydraulic cylinder seal failure, the lift cylinder will retract in an uncontrolled manner, with resultant boom retraction and load lowering.
Some booms incorporate measures to prevent unintended boom lowering in the event of hydraulic pressure loss to the boom""s lift cylinder(s). For instance, the Johnston ""327 patent discloses a locking mechanism including two sets of toothed cams arranged on opposites sides of the boom. The cams are configured to engage and lock an extended section of the boom to prevent unintended lowering of that section relative to the adjacent section upon the loss of hydraulic fluid pressure to the lift cylinder(s) for the extendible section. Each set of cams is mounted on a pivot shaft so that its cams are pivotable, 1) from a disengaged position permitting lowering of the extendible section relative to the adjacent section, 2) to an engaged position in which the teeth on the cams engage the extendible section to prevent it from retracting. The cams are normally pivoted into their disengaged position by a release cylinder that is supplied with system pressure. They pivot towards their engaged position under the force of a spring when the system suffers a pressure loss and the release cylinder is allowed to retract.
The hydraulic locking mechanism described in the Johnston ""327 patent operates very well in the event of a complete or near complete loss of hydraulic fluid pressure. However, a problem may arise if the system suffers only a partial pressure loss (on the order of less than 5% of the total system pressure). Such partial pressure losses can occur, for instance, when seepage occurs within the lift cylinders or when the volume of the pressurized fluid decreases as the fluid cools following a lift operation. In these situations, only enough fluid pressure is lost to result in sufficient release cylinder retraction to pivot the cams a few degrees. The cams of the two opposed sets seldom have exactly the same lost motion when pivoting from their disengaged position to their initial engaged position. Hence, when only a relatively small pressure loss occurs in the hydraulic system with a resultant relatively small cam stroke, it is entirely possible that only one set of cams will engage the extended boom section, while the other set of cams remains spaced from the extended section. In this situation, only one side of the extended section is locked, while the other side is still free to move unobstructed with respect to the adjacent section. A subsequent attempt to lower the boom under power of the lift cylinder will result in the imposition of uneven loads on the extended section that can lead to denting or even bending of the extended section. This potential problem can be avoided only by coordinating movement of the two sets of locking devices so that they are always either both engaged or both disengaged.
One way to achieve this effect might be to set overly large tolerances in the system such that neither set of cams would engage the extended boom section unless both sets pivot through a lost motion stroke indicative of complete or catastrophic pressure loss in the system. However, a system configured in this manner necessarily would exhibit a substantial delay between the time of the pressure loss and the time of cam engagement. This delay would prevent the locking mechanism from engaging the extended boom section before it begins to retract. This situation is undesirable because the ideal locking mechanism responds nearly instantaneously to system pressure loss so as to lock the extended section of the boom from retraction without any boom lowering and without imposing shock loads on the boom.
It is therefore a first principal object of the invention to provide an extendible boom incorporating a locking mechanism that avoids binding or bending loads on the locked boom section by assuring that locking forces are always applied at least essentially equally to both sides of the locked section.
A second object of the invention is to provide an extendible boom that meets the first principal object and that is relatively simple in construction, easy to fabricate, and easy to assemble.
Another object of the invention is to provide an extendible boom that meets the first principal object and that need not accommodate excessive lost motion in its locking mechanism.
In accordance with a first aspect of the invention, these objects are achieved by providing an extendible boom comprising a base, first and second extendible boom sections which are supported on said base, a hydraulic lift cylinder which has a lower end and which has an upper end operatively coupled to said first section so as to raise said first section relative to said second section upon lift cylinder extension, and a locking mechanism. The locking mechanism includes first and second locking devices which are mounted on opposite sides of the boom and each of which is movable, relative to the boom, a) from a disengaged position permitting lowering of the first section, b) to an engaged position in which the locking devices engage the first section to prevent the first section from being lowered. The locking mechanism additionally includes an equalizer arrangement which is connected to both of the first and second locking devices and which mechanically couples the locking devices to one another so that both of the locking devices always move through at least essentially the same stroke at least essentially the same time.
Preferably, each of the locking devices comprises at least one cam and a pivot shaft on which the cam is mounted for rotation therewith. In this case, the equalizer arrangement preferably comprises an equalizer bar having a first end operatively connected to the pivot shaft of the first locking device and a second end operatively connected to the pivot shaft of the second locking device.
A second principal object of the invention is to provide a method of lifting a load that is responsive to decreased hydraulic pressure in a hydraulic lift cylinder of an extendible boom to mechanically engage the extended boom section(s) without imparting any bending or twisting forces on the extended section.
Another object of the invention is to provide a method that meets the second principal object and that maintains near-immediate responsiveness to hydraulic pressure loss.
In accordance with another aspect of the invention, these objects are achieved by providing a boom having first and second sections, supporting the load on the first section, extending the first section relative to the second section using a hydraulic lift cylinder, and, in response to decreased hydraulic pressure to the lift cylinder, automatically mechanically engaging the first section to prevent unintended lowering of the load. The automatically engaging step comprises moving first and second locking devices on opposite sides of the first section into engagement with the first section while correlating movement of the first and second locking devices so that both of the locking devices always move through at least essentially the same stroke at least essentially the same time.
Preferably, the step of mechanically engaging comprises rotating a cam of each of the locking devices into engagement with the first section, the cams normally being retained in a disengaged position by hydraulic pressure and rotating into an engaged position upon loss of hydraulic pressure to the lift cylinder. Each of the cams may be mounted on a respective rotatable pivot shaft, in which case the pivot shafts are mechanically coupled together so that rotation of one pivot shaft drives the other pivot shaft to simultaneously rotate through a stroke that at least essentially equals a pivot stroke of the one pivot shaft.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description the specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.